1. Field of the Invention
The present invention is directed to a process for deposition of copper and, in particular, to an improved process and system for depositing metallic copper on a catalytically activated surface by controlled disproportionation of cuprous hydroxide.
2. Description of Related Art
The deposition of copper by means of disproportionation was disclosed in U.S. Pat. No. 3,963,842 to Sivertz et al., assigned to the assignee of the instant application. The patent described the electroless plating of copper onto silver, glass (preferably in manufacturing mirrors) or other catalytically activated surface by (1) rapidly reducing complexed cupric tetraamino ions in aqueous solution to complexed cuprous diamino ions, without substantial reduction to metallic copper and (2) thereafter effecting controlled disproportionation of the resultant cuprous ions to bring about the deposition of metallic copper principally on the surface. The initial reduction step was carried out by a class of reducing agents selected from nitrogenous materials containing one or two nitrogen atoms and having the formula: EQU X--NH--Y
wherein X is hydrogen, hydroxyl, hydroxyl-substituted lower alkyl or benzene sulfonyl groups and Y is --NH.sub.2. Specific members disclosed were hydrazine, salts of hydrazine with sulfuric acid or acetic acid, monosubstituted derivatives of hydrazine including 2-hydroxyethylhydrazine and p-hydrazino-benzene sulfonic acid, hydroxylamine, and salts of hydroxylamine with sulfuric acid or acetic acid. Co-reducers were also disclosed, including symmetrically disubstituted hydrazines used along with the hydrazine members, such as di-2-hydroxyethylhydrazine, hydrazo-benzene, and hydrazo-carbonamide, and other nitrogenous materials, for example, aminoguanadine bicarbonate. The use of hydroxylamine sulfate was preferred over the hydrazines.
In the preferred embodiment of this process it was believed necessary to use ammonium hydroxide in conjunction with hydroxylamine sulfate to convert copper from the +2 (cupric) oxidation state to the +1 (cuprous) form. The cuprous diammino ion would then disproportionate to copper metal and the cupric state when mixed with certain organic acid activators, such as hydroxy carboxylic acids, more preferably alpha hydroxy acids, such as glycolic, malic, tartaric, saccharic, citric and/or lactic acid and the like, dicarboxylic acids, such as succinic acid and the like, and sulfamic acid. Alternatively, an activator-modifier used for disproportionation may be a chelating amine such as ethylenediamine, triethyltetraamine, analogous alkylamines, or a mineral acid activator such as sulfuric or phosphoric acid with a modifier of said organic acids, said chelating amines or combinations thereof. Following disproportionation, copper in the metallic state would deposit on the glass, silver metal film or other activated surface.
An ammoniacal cuprous complex solution was also specified in a less successful process for depositing copper by disproportionation disclosed in U.S. Pat. Nos. 2,967,112 and 2,977,244.
Since its introduction in the mid-1970s, the aforementioned Sivertz et al. process has worked well in general and has been licensed and used by numerous mirror manufacturers around the world. However, there are certain aspects of the process in which improvement would be welcomed. The ammonia-cuprous complex used in the aforementioned process would be destroyed within minutes if exposed to air so that it generally had to be formed in an enclosed container. The only way the cuprous complex could be saved was if a nitrogen blanket was placed over it, or if it was otherwise shielded from contact with oxygen, which was not always practical. The metallic copper film deposited using the Sivertz et al. process also has a tendency to tarnish upon exposure to water vapor, especially when fugitive ammonia vapors from the process are present, which complicates protection during processing. The reaction rate of the process has also not been considered to be amenable to control. Furthermore, manufacturers are always looking for improved reaction efficiency to conserve material costs and speed process time. Thus, there has been a long-felt need for improvement of the process of the Sivertz et al. patent.
Recently, environmental regulations in the U.S. and elsewhere in the world 3,5 are beginning to regulate the discharge of ammonia into water sewers and streams, and in the workplace environment. Compliance with such regulations are requiring companies to eliminate the use of ammonia altogether in their processes, or to install expensive pollution control equipment to eliminate the presence of ammonia in the effluent stream and the workplace. Conventional technology and best available equipment that could be used to remove ammonia from an effluent stream is costly and not very efficient. Air stripping merely places the ammonia in the air and does not eliminate a pollution product. The destruction of ammonia can also be accomplished by the use of chlorine, but that system results in the formation of chlorinated organic compounds which present potential pollution and health hazards. Also, the preferred process has utilized hydroxycarboxylic acids such as citric acid in the activator-modifier, but has required use of such organic acid in an amount which causes a significant chemical oxygen demand and biological oxygen demand upon the effluent. Use of a significantly lower amount of citric or other organic acid would help from a pollution standpoint to control the amount of undesirable reaction products.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved process for depositing copper which eliminates the use of ammonia, and the associated problems entailed with controlling and removing ammonia in the process.
It is another object of the present invention to improve the efficiency of the aforedescribed Sivertz et al. process for depositing copper.
It is a further object of the present invention to provide a method for controlling the reaction rate of the aforedescribed Sivertz et al. process for depositing copper.
It is yet another object of the present invention to improve the aforedescribed process for depositing copper by reducing the amount of organic acid needed for disproportioning the cuprous ions.
It is a further object of the present invention to provide an electroless deposited copper coating film that does not readily tarnish upon exposure to water vapor.
It is yet another object of the present invention to improve the aforedescribed process for depositing copper by eliminating the need for a nitrogen blanket over the cuprous complex or otherwise holding the cuprous complex in an enclosed container.
It is a further object of the present invention to provide a cuprous complex for use with the aforedescribed process for deposition of copper in which the cuprous complex solution can be prepared and easily held or stored for relatively long periods of time prior to applying the copper film to the desired substrate by disproportionation.
It is yet another object of the present invention to provide a system of compositions for achieving the aforedescribed objects.